Electronic Structure and Transport in the Potential Luttinger Liquids CsNb$_3$Br$_7$S and RbNb$_3$Br$_7$S

The crystal structures of ANb$_3$Br$_7$S (A = Rb and Cs) have been refined by single crystal X-ray diffraction, and are found to form highly anisotropic materials based on chains of the triangular Nb$_3$ cluster core. The Nb$_3$ cluster core contains seven valence electrons, six of them being assigned to Nb-Nb bonds within the Nb$_3$ triangle and one unpaired d electron. The presence of this surplus electron gives rise to the formation of correlated electronic states. The connectivity in the structures is represented by one-dimensional [Nb$_3$Br$_7$S]$^-$ chains, containing a sulphur atom capping one face ($\mu_3$) of the triangular niobium cluster, which is believed to induce an important electronic feature. Several types of studies are undertaken to obtain deeper insight into the understanding of this unusual type of material: the crystal structure, morphology and elastic properties are analysed, as well the (photo-) electrical properties and NMR relaxation. Electronic structure (DFT) calculations are performed in order to understand the electronic structure and transport in these compounds, and, based on the experimental and theoretical results, we propose that the electronic interactions along the Nb chains are sufficiently one-dimensional to give rise to Luttinger liquid (rather than Fermi liquid) behaviour of the metallic electrons.